Update README.md

Author: mattpolzin

README.md

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 # JSONAPI+OpenAPI
 The `JSONAPIOpenAPI` framework adds the ability to generate OpenAPI compliant JSON documentation of a JSONAPI Document.
 
+See the Open API Spec here: https://github.com/OAI/OpenAPI-Specification/blob/master/versions/3.0.2.md
+
 *This library is in its infancy. The documentation will grow as the framework becomes more complete.*